1. Field of the Invention
The present invention relates to a display device which has an organic light emitting element and to a method of driving the display device. Specifically, the present invention relates to a display device in which an organic light emitting element is driven with alternating current and a driving method thereof.
2. Description of the Related Art
In recent years, a technique of forming a transistor, for example, a TFT (thin film transistor) on a substrate has been advanced greatly to promote development of active matrix display devices. Active matrix display devices are free from the problem of cross talk in passive matrix display devices and therefore are capable of displaying images with higher definition and contrast compared to passive matrix display devices.
TFTs using polysilicon for their active layers have higher field effect mobility than conventional TFTs that use amorphous silicon and therefore can operate at high speed. This makes it possible to perform control the luminance of pixels, which has conventionally been done by an external driving circuit to the substrate, by a driving circuit formed on the same substrate on which the pixels are formed. Thus formed active matrix display devices can have various circuits and elements on the same substrate, which helps to reduce the display devices in size.
A display device using organic light emitting elements (hereinafter referred to as organic light emitting displays) has also been developed actively in recent years. An organic light emitting elements is self-luminous and does not need a light source such as back light, unlike liquid crystal display devices. Therefore, organic light emitting elements is deemed as promising means for reducing weight and thickness of display devices, and are expected to be used in cellular phones, personal portable information terminals (personal digital assistant: PDA), and the like.
An organic light emitting element has a diode structure with an organic compound layer sandwiched between two electrodes and emits light by re-combining electrons injected from one of the electrodes with holes injected from the other electrode in an organic compound layer. The organic light emitting element provides electro-luminescence (EL), for example, fluorescence or phosphorescence. Because of the diode structure, the organic light emitting elements is also called organic light emitting diodes (OLEDs).
An organic light emitting element is often composed of an anode, a hole transporting layer, a light emitting layer, an electron transporting layer, and a cathode. This structure is so high in light emission efficiency that almost all organic light emitting elements that are under development at present employ this structure. One or both of hole injection layer and electron injection layer are omitted in some cases. However, a light emitting layer is an indispensable component to an organic light emitting element since an organic light emitting element emits light through recombination of carriers when current flows into the light emitting layer.
An organic compound layer is a collective name for carrier (electrons and holes) transporting layers, a light emitting layer that is formed of a material with high quantum yield, and other layers. The light emitting layer, hole injecting layer, and electron injecting layer mentioned above are included in the organic compound layer.
An organic light emitting element has a high rectifying characteristic and emits light through recombination of carriers when the electric potential of its anode exceeds the electric potential of the cathode to cause current to flow in the organic compound layer. On the other hand, when the electric potential of the anode is lower than the electric potential of the cathode, current does not flow in the organic compound layer and no light is emitted. In the diode structure as an organic light emitting element, voltage applied in a direction, in which current readily flows is called forward bias whereas voltage applied in a direction in which current finds it difficult to flow, is called reverse bias.
FIG. 19 shows an equivalent circuit of a pixel portion in a conventional active matrix organic light emitting display.
Gate signal lines (G1 to Gn) are connected to gate electrodes of switching TFTs of pixels. Each switching TFT is denoted by 901 and each pixel is denoted by 900. Each switching TFT has a source and a drain, one of which is connected to one of source signal lines (S1 to Sn) for inputting data signals and the other of which is connected to a gate electrode of a current controlling TFT 902 and to one of electrodes of a capacitor 903. Each pixel has one current controlling TFT 902 and one capacitor 903. The other electrode, namely, one of electrodes of a capacitor that is not connected to a switching TFT is connected to one of power supply lines (V1 to Vm).
Each current controlling TFT has a source and a drain, one of which is connected to one of the power supply lines and the other of which is connected to a pixel electrode of an organic light emitting element 905. Each pixel has one organic light emitting element 905. An opposite electrode of an organic light emitting element faces the pixel electrode of the organic light emitting element and sets the reference electric potential for the pixel electrode.
For conveniences' sake of explanation, an opposite electrode is connected to an opposite power supply 906. The difference in electric potential between the power supply lines and the opposite power supply is set to a level at which the voltage is large enough to cause an organic light emitting element to emit light.
Each organic light emitting element 905 has an anode and a cathode, one of which serves as a pixel electrode and the other of which serves as an opposite electrode. An anode of an organic light emitting element serves as a pixel electrode and a cathode of the organic light emitting element serves as an opposite electrode when the anode of the organic light emitting element is connected to a source or drain of a current controlling TFT. On the other hand, a cathode of an organic light emitting element serves as a pixel electrode and an anode of the organic light emitting element serves as an opposite electrode when the cathode of the organic light emitting element is connected to a source or drain of a current controlling TFT.
The luminance of light emitted from an organic light emitting element is determined as follows. A selection signal is inputted from the gate signal line to the gate electrode of the switching TFT 901 to turn the switching TFT ON (conductive). Then, a data signal inputted to the source signal line is inputted to the gate electrode of the current controlling TFT 902 through the switching TFT. The electric potential of the gate electrode of the current controlling TFT is held by the capacitor 903. Accordingly, the difference in electric potential between the gate electrode of the current controlling TFT 902 and the power supply line (one of V1 to Vm), namely, the gate voltage of the current controlling TFT is kept constant until a next data signal is inputted to the pixel.
When the current controlling TFT is turned ON, current flows from a semiconductor layer of the current controlling TFT to the organic light emitting element that is connected in series to the semiconductor layer of the current controlling TFT. The intensity of light emitted from the organic light emitting element is determined in accordance with the amount of current that flows into the organic light emitting element. The amount of current flowing into a current controlling TFT is controlled by a data signal inputted to each pixel and therefore the luminance of light emitted from each pixel can be controlled by the electric potential of data signal.
In general, ‘driving an organic light emitting element with direct current’ means maintaining light emission by keeping the electric potential of an anode higher than the electric potential of a cathode to cause direct current to flow. In this case, the anode is either an opposite electrode or a pixel electrode and the cathode is the other of the two electrodes.
However, when an organic light emitting element is driven with direct current, the luminance of light emitted from the organic light emitting element is lowered with time. Direct current driving causes degradation of luminance with age supposedly because ionic impurities accumulate in the interface of the organic compound layer and molecules constituting the organic compound layer are polarized in a single direction along the electric field to create inside the organic compound layer an electric field reversely oriented with respect to an electric field applied by the pixel electrode or the opposite electrode of the organic light emitting element.
In particular, the luminance of light emitted from an organic light emitting element is markedly lowered with time if the organic light emitting element is driven while the voltage applied between the cathode and anode of the organic light emitting element is kept constant (hereinafter referred to as constant voltage method). In the constant voltage method, since the level of the voltage applied to the cathode and the anode is always constant, the effective voltage applied to the organic compound layer is reduced as the intensity of the electric field created in the organic compound layer increases to lower the luminance of light emitted from the organic light emitting element.
In order to prevent this degradation of luminance, the organic light emitting element has to be driven with alternating current. ‘Driving an organic light emitting element with alternating current’ means alternately applying voltages with different polarities to the organic light emitting element. In other words, reverse bias is applied in addition to forward bias that is necessary for light emission. Forward bias and reverse bias may not always have the same magnitude or application time. When a minute reverse bias is applied, it still is regarded as alternating current driving.
However, in the conventional circuit described above, connecting an alternating current source to the opposite electrode of the organic light emitting element and to the power supply lines is sometimes not enough to apply a sufficient reverse bias to the organic light emitting element. This will be explained below.
The circuit shown in FIG. 19 is a closed circuit in which the source or drain of the current controlling TFT 902 is connected serially to the organic light emitting element 905 between the opposite power supply 906 and the power supply line (one of V1 to Vm). The operation of this closed circuit in applying voltages to the organic light emitting element with alternating current is described. To simplify the explanation, the current controlling TFT in the description below is a p-channel TFT. The p-channel TFT is turned ON when the electric potential of the gate is lower than the electric potential of the source, namely, the electric potential of the power supply line and the difference thereof exceeds the threshold.
Therefore, in order to apply forward bias to the organic light emitting element, the electric potential of the opposite electrode of the organic light emitting element 905 is set to the Lo level whereas the electric potential of the power supply line is set to the Hi level, and the electric potential of the gate of the current controlling TFT is lower than the electric potential of the power supply line (the source of the current controlling TFT) to exceeding the threshold. The current controlling TFT is turned ON, namely, conductive, and current flows into the organic light emitting element 905 to cause the organic light emitting element to emit light.
When the current flowing into the organic light emitting element is to be stopped without changing the electric potentials of the power supply line and opposite electrode, the electric potential of the gate of the current controlling TFT is set higher than the electric potential of the source thereof to turn the current controlling TFT OFF.
Suppose that now the current controlling TFT is ON and the electric potentials of the power supply line and opposite electrode are inverted to apply reverse bias to the organic light emitting element. That is, the electric potential of the opposite electrode of the organic light emitting element 905 is set to the Hi level whereas the electric potential of the power supply line is set to the Lo level. At this point, the series circuit consisting of the opposite electrode, the organic light emitting element, the current controlling TFT, and the power supply line is equivalent to a source follower circuit. Accordingly, a great reduction in voltage occurs here since the gate electric potential of the current controlling TFT is low, and therefore only insufficient reverse bias is applied to the organic light emitting element.
As has been described, outputting an alternating current waveform between the opposite power supply 906 and the power supply line is sometimes not enough to apply a sufficient reverse bias to the organic light emitting element. The same applies to the case where the current controlling TFT is an n-channel TFT in order that current flows from an anode to a cathode in an organic light emitting element when a pixel electrode of the organic light emitting element serves as the cathode and a opposite electrode thereof serves as the anode.
It is thus difficult to drive an organic light emitting element with alternating current in the conventional circuit structure described above, and the problem of great reduction in luminance of an organic light emitting element with time due to application of direct current voltage is yet to be solved.